Grazing incidence spectroscopic methods and apparatus



July 28, 1970 B. s. FRAENKEL ETA| 3,521,957

GRAZING INCIDENCE SPEGTROSGOPIC METHODS AND APPARATUS Filed Sept. 1, 1965 2 3 1 4 6 w DISTANCE (m I Puma.

T w \E INVENTORS x Wm WM |x BY F I G 4 W ATTO RN EYS United States Patent US. Cl. 356-79 5 Claims ABSTRACT OF THE DISCLOSURE A real and inverted image of every spectral line recorded upon the film plate of a grazing incidence spectrometer is provided by viewing the complete length of the light source employed and including an adjustable slit perpendicular to the main slit of the spectrometer between the main slit and a diifracting unit.

This invention relates to spectroscopic methods and to apparatus for effecting the methods.

The invention is generally useful in the spectroscopic investigation of excited ions, atoms, and molecules in light sources and, in particular, is valuable for research into the temperature and matter distribution in hot stars, for example.

By combining the spectrometer with the pinhole camera, in accordance with one aspect of the invention, there is obtained not only the customary dispersion, along the length of the photographic plate of the spectrum for the different Wavelengthssuch as is provided by a conventional spectrometerbut also a real and inverted image of the light source produced by the pinhole camera. Every spectral line (in a line spectrum) thus recorded on the plate, will be a real and inverted image of the spatial distribution of the excited ions, atoms, or molecules, the emission of which causes that line. That is, the variation in exposure of the plate along the length of the line (which determines the profile of the line) is an image of the distribution of those particular ions in the source.

As used hereafter, the word ion is intended to embrace the terms atoms and molecules.

Every ion emits a large number of spectral lines, and approximately all of them will have a similal profile on the plate, although the intensity, that is, the blackness of the line on the light sensitive emulsion of the plate, for reasons of the transfer probability, will be dilierent for each line.

If the light source contains a number of different kinds of ions (ions at various stages of ionization or ions of various metals) and their distribution in the light source varies, then the profile of the spectral lines belonging to the various ions will also vary as a function of the elevation across the width of the plate. It is possible to obtain an idea of the distribution of ions of the same degree of ionization in the light source from the shape of the profile of a line belonging to a known degree of ionization. In addition to this, even if only a few of the lines belonging to a particular degree of ionization are known, it is possible to determine all the other lines having the same degree of ionization. This is done by grouping the lines according to their elevation across the plate. By elevation is meant the height of that point in a line that has most strongly exposed the plate.

If the density of the ions in the light source is not uniform, the image of the regions of high ion density will be more intense, because the photographic plate has received more light, than the image of the regions of low 3,521,957 Patented July 28, 1970 'ice ion density. By studying the intensity variations of the spectral line over its length, it is possible to gain an idea of the density distribution of the ions producing that line in the light source.

Thus, in accordance with the invention, one is enabled to determine the distribution of the density of ions in a light source and, as the case may be, the distribution in the light source of ions of different degrees of ionization, as well as to group all lines having like degrees of ionization.

An embodiment of the invention will now be described in detail, with reference to the figures, wherein:

FIG. 1 is a schematic representation of a pinhole camera;

FIG. 2 is a schematic representation of the invention as applied to a grazing incidence spectrometer;

FIG. 3 is a normalized representation of the distribution of ions of dilferent degrees of ionization in a light source; and

FIG. 4 illustrates qualitatively the spectral line images obtained by the invention.

Referring to FIG. 1, by way of introduction to the invention, a pinhole camera is illustrated wherein reference numerals 1, 2, and 3 respectively designate the object, a mask having therein a pinhole 4, and the real, inverted image. From elementary geometry it will be apparent at once that the magnification will be b/a where b and a respectively are the distance from the image and from the object to the plane of the pinhole.

Referring now to FIG. 2, the invention is illustrated as applied to a grazing incidence spectrometer, wherein reference numeral 5 designates the customary main slit, 6 designates an auxiliary slit for limiting the width of the light-which slit may or may not be usedthe grating 7, the light-sensitive plate 8, and the light source 9, having a positive and a negative electrode 10 and 11, respectively. The beam 14 passes successively through the slits, onto the grating 7, and finally onto the plate 8.

In addition to the main slit 5 and the auxiliary slit 6, the invention contemplates a third slit 12, located between the diilracting element (in the particular example the grating 7) and the main slit 5, the length of which is perpendicular to that of the main slit. In the case where an auxiliary slit is employed, the slit 12 will lie between the main and the auxiliary slits, as shown in the figure.

The third slit 12, which is the heart of the invention, is made movable between the two limiting positions comprised of the slit 5 on the one hand, and the slit 6 or the diffraction element 7, on the other hand, by any suitable and known means shown schematically at 13.

The function of slit 12 is to act as the aperture of a pinhole camera. Inasmuch as we are interested in imaging only in the longitudinal direction of the main slit, the invention employs a slit, instead of a pinhole, running perpendicular to that direction. The distribution of radiating ions of various degrees of ionization, as a function of the distance in the light source measured along a line parallel to the length of the main slit, is diflferent for every degree of ionization.

FIG. 3 illustrates, in a qualitative way, such distributions for the sixth, seventh, eighth, and ninth spectra of iron, as measured from the positive electrode of the source. The highest points of the distributions are normalized. With the arrangement of the invention, the densest point of blackening (the point of strongest exposure) along a spectral line is obtained as an inverted, real image of the densest distribution point of the ions of the degree of ionization belonging to that line. Thus, the lines of ions of different ionizations have different elevations on the photographic plate.

As is known, each spectral line is associated with a fixed degree of ionization of the atom or molecule. The atoms or molecules which are excited in the light source, upon falling to a lower energy state, emit electromagnetic radiation of wavelength characteristic of that transition.

FIG. 4 illustrates qualitatively what is obtained on a photographic plate, employing the method and the apparatus of the invention, when the distribution of ions, in the source, of different degrees of ionization are as shown in FIG. 3. Because the maxima of the distribution curves in FIG. 3 have been normalized, as previously indicated, each of the spectral lines in FIG. 4 is shown as having the same width at its elevation E, which generally will not be the case. It Will be noted, comparing FIGS. 3 and 4, that the elevation E of each line is characteristic of its degree of ionization and that the profile of each line varies in accordance with the distribution curve of the ions, of that degree of ionization in the light source.

In practicing the invention, slits 5 and 6 are made sufliciently long so as not to mask the light source, and thus are longer than ordinarily used. Since no mask is placed in front of the plate, the spectral lines are recorded in their full length. The length of slit 12 generally will exceed 2 millimeters.

The widths of slits 5, 6 and 12 are determined by the desired resolving power, in accordance with the general rule that the narrower the slits the greater the resolving power. In particular, the width of slit 12, which may vary from about 0.2 to 2.0 mm., will be determined by considerations of resolution and intensity, for as the resolution is improved the intensity of the light incident upon the plate is reduced.

By way of example, but not in limitation thereof, one embodiment of the invention, FIG. 2, had the following parameters and specifications:

(a) The light source 5 was a triggered vacuum spark, described by G. Ballofet in Ann. Phys. 5, 1256 (1960), on page 1249, arranged such that the axis passing through the electrodes was parallel to the length of slit 5; the separation between the source and the main slit was varied as desired;

(b) The lengths of the light source 9 and the slits 5 and 6 were, respectively, 7 mm., 10 mm., and 10 mm.; the length of slit 12 was greater than 2. mm.;

(c) The widths of slits 5, 6 and 12 were, respectively, 0.005 mm., 2.0 mm., and 0.2 mm.; and

(d) The spacings A, B, C and D were 230 mm., 174 mm., 96 mm., and 34 mm., where, in each case, the distance is measured from the center of the diffraction element 7 to the light source 9, the plane of slit 5, the plane of slit 12, and the plane of slit 6, respectively, as shown in FIG. 2.

The magnification of the spectral line of wavelength A in its longitudinal direction will be:

where:

C is, once again, the distance from the center of the diffraction element 7 to the plane of the slit 12;

F is the distance between the center of the diflraction element and the point at which the light of spectral line of wavelength strikes the photographic plate 4; and

G is the distance of separation between the light source 9 and the slit 12.

By changing the position of slit 12, the elevation of any given line can be changed, as desired, or the separation between elevations of lines of different ionization can be magnified or, of course, reduced at will. To increase the separations, the slit is moved toward the source, thereby simplifying the task of distinguishing lines of the same degree, or between lines of difierent degrees, of ionization.

Although a particular embodiment of the invention has been described and illustrated as applied to a grazing incidence spectrometer, the invention, in fact, is applicable to spectrometers generally for the investigation of continuous spectra or discontinuous, line, emission spectra of excited atoms, ions, or molecules. Similarly, the choice of light source is not limited by the invention, but is determined solely by the nature of the investigation, and may be terrestrial, such as a spark, or extraterrestrial, such as a hot star, since the image formed is characteristic of the source; and the qualification light is not to imply that the radiation of the source is limited, in whole or in part, to the visible spectrum.

What is claimed is:

1. In combination with a grazing incidence spectrometer receiving light in the grazing incidence region from a source and having a main slit irradiated by said source, a light diifracting means downstream of said slit, and a light sensitive emulsion downstream of said diffracting means, said spectrometer permitting the resulting spectrum to be recorded in its full height: elongated slit means located between said main slit and said ditfracting means for real imaging on said emulsion said source in the longitudinal direction of said main slit, said elongated slit lying substantially transverse of the light path of the grazing incidence spectrometer and extending substantially perpendicular to the longitudinal direction of said main slit.

2. The combination of claim 1, including means for permitting movement of said elongated slit means back and forth between said main slit and some point upstream of said diifracting means, whereby the magnification in said longitudinal direction of the real image can be varied in dependence on the position of said imaging means.

3. The combination of claim 2, wherein said spectr0rneter includes an auxiliary slit located between said elongated slit means and said diffraction means, said auxiliary slit lying transverse of the light path of the spectrometer and extending in a direction parallel to the main slit, said main and auxiliary slits being of a length sufiicient to pass the complete length of said light source.

4. The combination of claim 3 wherein said imaging slit being greater than 2 millimeters in width and having a length in the range of from 0.2 to 2 millimeters.

5. The combination of claim 3 wherein said elongated slit is greater than 2 millimeters in length and has a width in the range of from 0.2 to 2 millimeters.

References Cited UNITED STATES PATENTS 2,431,734 12/1947 Cutting 35698 2,179,657 11/1939 Estey.

FOREIGN PATENTS 9/1961 Canada.

OTHER REFERENCES RONALD L. WIBERT, Primary Examiner V. P. MCGRAW, Assistant Examiner U.S. Cl. X.R. 

